A ducted fan is a mature propulsion technology that offers a number of advantages over non-ducted fans and propellers in terms of efficiency, noise and safety. As such, ducted fans have been used in a variety of manned and unmanned aircraft, airboats and hovercraft.
Low aspect ratio ducts, those with large chords with respect to their diameters, are often employed in craft designed to transition from powered vertical flight to horizontal flight and back, as the large chord of the duct adds wing area in horizontal flight. In addition, low-aspect ratio ducts also accommodate counter-rotating propellers or stator blades that reduce the swirling flow coming off the propeller, thereby straightening the airflow before it exits the duct. Straightening the exiting airflow enhances both thrust and efficiency. While there is drag associated with placing stator blades in the fan downwash, this effect may be partially offset through stator shaping.
In contrast, the airflow from high aspect ratio ducts, i.e., those with very short chords with respect to their diameters, typically exhibit high levels of swirl that are not easy to counter since the short chord makes it difficult to mount effective stators or counter-rotating propellers within the duct. The swirl from such a ducted fan can present a number of problems, especially those related to ground effect. These problems may be further exacerbated if the fan uses conventional control surfaces. The advantage, however, of high aspect ratio ducts is that the static thrust of the fan or propeller is increased while the momentum drag at high forward speeds may be reduced.
Ejector technology is a mature aerodynamic concept in which the thrust of a jet stream is increased by increasing the momentum flow through entrainment. Unfortunately, the increased thrust is typically gained at the expense of weight and drag and the volumetric penalties incurred in installation. Additionally, ejectors generally require long diffuser and mixing sections in order to achieve adequate augmentation, thus making them difficult to integrate into a realistic aircraft configuration.
Initial research on ejector technology focused on understanding and increasing the available augmentation. It was found that the mechanism of entrainment that leads directly to augmentation could be improved through the introduction of vortices, i.e. spinning flow. More recently, investigators have discovered the potent effect of pulsating flow or “pumping” on turbulent mixing efficiency, which is now known to be one of the predominant mechanisms of ejector effectiveness. While initial studies indicated that augmentation could be optimized with pumping frequencies of approximately 110-135 Hz, subsequent research demonstrated that even greater gains could be achieved by selectively tuning the pulse frequency to match the characteristics of the ejector cavity. The issue associated with pumping is that most ejector applications use a jet stream as the primary flow, making it difficult and intrusive to oscillate.
In powered-lift aircraft, the primary thrust vector is generally pointed vertically toward the ground. This implies that when a ducted fan is employed for primary lift, the lifting plane is essentially parallel to the horizon. The general control philosophy is to modify the thrust vector of the ducted fan in order to control the direction of the vehicle. To change altitude is therefore a simple matter of varying engine throttle or a collective change of fan blade pitch. It is much more difficult, however, to exert control forces in the remaining degrees of freedom.
To move fore, aft or laterally with respect to the horizon, or in the lift plane of the ducted fan, is to typically tilt the duct and rotate its thrust to “push” the fan in the desired direction. The significance of this technique is that lateral forces are created by tilting, and the tilting is accomplished by generating rotational moments whose axis is in the plane of the duct normal to the lateral direction. Accordingly, generating control forces is actually a problem of generating rotational moments.
Due to the complexity of articulated rotors, most ducted fan aircraft employ a fixed pitch propeller and aerodynamic control surfaces mounted within either the downwash or the inlet side of the duct to provide control moments in all three axes. As a consequence of this approach, at times the control surfaces are required to induce multiple control moments at once, e.g., yaw and rolling to provide a coordinated turn. This, in turn, can lead to the saturation of the control surfaces, an issue that must be taken into account during design and operation of the vehicle in order to avoid reduction or loss of control authority during flight.
In order for control surfaces mounted in the downwash to be effective, preferably they are placed between 1.5 and 2.0 rotor diameters behind the duct exit plane. Therefore, a vehicle's height may be significantly increased. While such a height increase may be acceptable for small diameter fans, with larger diameter fans such a configuration is impractical.
Ground effects introduce yet another issue relating to the use of aerodynamic control surfaces in a ducted fan vehicle. More specifically, the interaction of the downwash with the ground impinges on the control surfaces at angles different from those encountered during normal flight, potentially rendering the surfaces ineffective by reducing or reversing their moments. To reduce this effect, many ducted fan vehicles stand tall on their gear to keep the control surfaces in clean air and minimize ground effects. Alternately, the vehicles may use a specially designed launch platform that limits the adverse flight characteristics that may be present near the ground.
Another subtlety associated with aerodynamic control surfaces occurs when they are used with a high-aspect ratio duct. In this case, the inherent swirl of the downwash into which the control surfaces are placed limits their effective range of angle of attack, as small changes can stall portions of the surface.
One approach that has been used successfully to counter these effects is to place the control surfaces above the ducted fan on the inlet side where the flow is still generally perpendicular to the lift plane. This technique is equally effective for high and low-aspect ratio ducts, as the inlet flow is devoid of swirl. The downside to mounting the control surfaces in the inlet side is that due to the slower airflow above the duct, larger control surfaces are required in order to provide the desired level of control. This effect can be managed, to a degree, by lowering the control surfaces into the duct where the inlet velocity more closely matches the outlet velocity.
A bigger issue associated with placing the control surfaces at the duct inlet is coordination between the control surfaces and the vehicle center-of-gravity (CG), such coordination being required to avoid the introduction of control coupling. Control coupling occurs when a desired output (e.g., roll right) is accompanied by an unintended response (e.g., pitch) that must be compensated for by the pilot or control system. Coordination between the control surfaces and the CG is complicated by the fact that the CG changes during flight and vehicle operation, for example due to fuel use or payload being loaded or removed. As the CG moves away from its initial position, the moment arm to the lifting plane changes length accordingly, changing the coupling moment in pitch or roll. Worse, if the CG were to move above or below the lifting plane from its initial position, the direction of the moments would reverse. Due to these issues, control surfaces in the duct inlet are primarily used for yaw control as yaw control is less affected by vehicle CG and the associated aerodynamic forces can be balanced radially across the duct or ducts.
Although a variety of different approaches have been studied to solve these control issues, to date they have met with limited success. Accordingly, what is needed is an improved control system for use with a ducted fan. The present invention provides such a system.